Tiahleigh Palmer told her foster parents their son, 19, had been sexually assaulting her when she was left alone with him in the days before her foster father killed her, a court has heard.

Trent Thorburn, 19, allegedly admitted to a cousin via Facebook that he had incestuous sex with his 12-year-old foster sister and feared he impregnated her on October 28, two days before she was believed to have gone missing from Marsden State School.

Police struggled to identify exactly how Tiahleigh died because her body was found severely decomposed in a river near the family home in Logan, Queensland.

Investigators believe she was murdered in her family home the day before Rick Thorburn, 56, told police he had dropped her safety to school on October 30.

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Tiahleigh Palmer told her foster parents their son, 19, had been sexually assaulting her when she was left alone with him in the days before her foster father killed her, a court has heard

Prosecutors claim she had told her foster parents, Julene (left) and Rick Thorburn (right), that their son had been preying on her when they left the house before she was murdered

Trent Thorburn, 19, allegedly admitted to a cousin via Facebook that he had sex with his 12-year-old foster sister and feared he got her pregnant on October 28

Trent has been charged with incest, perjury and perverting the course of justice, while his father, 56, has been charged with murder and interfering with a corpse.

The court heard Tiahleigh had told a friend that she went to her foster mother Julene Thorburn, 53, to report the sexual abuse, the Australian reported.

'The victim had disclosed to her that when her foster carer went out, their son would then have sex with her,' police prosecutor Ellen French told the court.

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'She further said that the victim claimed to have disclosed this to her foster mother.'

Ms French said a friend of Trent's also told police the young girl had told her foster parents: 'Trent tried to touch me.'

The court heard Trent denied the allegations to his parents and on five other occasions to police, but eventually admitted he had lied about the sexual abuse.

Rick Thorburn (left), his wife Julene (middle right) and two sons, Josh (middle left) and Trent (right) have been charged with a string of offences in relation to the death of Tiahleigh Palmer

Prosecutors believe Tiahleigh was killed by her carer Rick Thornburn (pictured) at the family home a day before he claimed to have dropped her at school

Further details about the conversation Trent had with his cousin on Facebook messenger about his sexual activity with Tiahleigh were also revealed in court, the Australian reported.

'During that conversation he said he had sex with the victim on the prior Monday and he had only done so because she had threatened to kill his dog,' Ms French said.

'The defendant said he was concerned about being pregnant even though he had pulled out,' she said, adding that he claimed to have told his mother about it.

A sexual relationship between siblings, including under foster, step or de facto circumstances, is considered incest under the Queensland Criminal Code and can attract a maximum life sentence.

Police allege that after learning of his son's alleged actions and Tiahleigh's suspected pregnancy, he feared the bisexual dancer would become a target for violence offenders in prison had he been convicted.

Trent Thorburn, 19, is alleged to have told a cousin in a Facebook message that he had slept with his foster sister Tiahleigh Palmer and was concerned she was pregnant

CHARGES LAID OVER TIAHLEIGH PALMER'S MURDER

FOSTER FATHER RICK THORBURN: Charged with murder and interfering with a corpse. The 56-year-old was due to face court on Wednesday but is in an induced coma after collapsing while in custody.

FOSTER BROTHER TRENT THORBURN: Charged with incest, attempting to pervert the course of justice and two counts of perjury. The 19-year-old was denied bail on Wednesday after a court heard he had admitted on Facebook Messenger to having sex with Tiahleigh and was worried she might be pregnant.

FOSTER BROTHER JOSH THORBURN: Charged with perjury and attempting to pervert the course of justice. The 20-year-old is on bail and due to face court on October 10.

FOSTER MOTHER JULENE THORBURN: Charged with perjury and attempting to pervert the course of justice. The 54-year-old is on bail and due to face court on October 10.

In opposing bail, Ms French said Thorburn had obstructed police during the investigation and that he and his family had betrayed Tiahleigh's trust.

'The victim was put in care and protection of the defendant and his family,' Sgt Elen French told the court.

'This defendant betrayed her trust. He abused his position as big brother and took advantage of her.

'A consequence of that is her young life has been taken.'

According to the Courier Mail, the perjury charges relate to the family providing false testimony to the Crime and Corruption Commission during a secret hearing relating to Tiahleigh's death earlier this year.

Thorburn's lawyer argued he would serve more time in custody than if he were sentenced on the charges if bail were refused, and his client was young and had no criminal history.

He was denied bail.

Thorburn faced a Brisbane court on Wednesday after being charged with incest, perjury and attempting to pervert the course of justice over Tiahleigh's death

Tiahleigh's body was located in November last year on the banks of the Pimpama River on the northern Gold Coast

Tiahleigh's foster father, 56, was due to appear in the same court on Wednesday but was in an induced coma in hospital after a suspected self-harm attempt while in custody.

He was a pallbearer at her funeral.

Thorburn's wife Julene, 54, and another son, Josh, 20, have been charged with perjury and attempting to pervert the course of justice.

They have been released on bail and will face court on October 10.

The charges comes 11 months after Tiahleigh's naked and decomposing body was found by fishermen dumped by the banks of the Pimpana River.

She was gone for six days before she was registered missing by police and an alert was issued about her disappearance.

Police charged Trent on Tuesday with incest, perjury and attempting to pervert the course of justice over Tiahleigh's death

Trent Thorburn, 19, allegedly sexually assaulted his foster sister Tiahleigh Palmer during the 10 months she had been in his Queensland family's care last year